1. Field of the Invention
The present invention relates in general to a vaporizer apparatus used for growing thin films from a liquid feed material, and relates in particular to a liquid feed vaporizer apparatus suitable for growing high dielectric or ferroelectric films such as barium/strontium titanate.
2. Description of the Related Art
In recent years, there has been a remarkable improvement in circuit density of integrated circuit devices produced by the semiconductor industries, and intense development activities are underway in anticipation of giga-bit order dynamic random access memories (DRAMS) replacing the prevailing mega-bit order DRAMs of today. As dielectric thin film materials used to make high capacitance devices necessary for such highly integrated DRAMs have, newer materials such as barium titanate (BaTiO.sub.3) or strontium titanate (SrTiO.sub.3) of dielectric constant about 300 or mixtures of these compounds are deemed to be promising, instead of the present dielectric thin film materials including silicon oxide or silicon nitride films of dielectric constant less than 10, tantalum pentaoxide (Ta.sub.2 O.sub.5) films of dielectric constant less than 20. Materials with even higher dielectric constants such as PZT, PZLT and Y1 are also promising.
Of the various methods of making such thin films, prospects are particularly bright for the chemical vapor deposition (CVD) process, and in this case, it is necessary that a feed gas must be supplied in a stable gas stream to a substrate disposed in a film deposition chamber. In order to stabilize the vaporization characteristics, the feed gas is derived by heating for vaporization a liquid source produced by dissolving materials such as Ba(DPM).sub.2 or Sr(DPM).sub.2, which are solid at normal temperature, in some organic solvent such as THF.
It is extremely difficult to stably vaporize feed materials of highly dielectric materials described above because: (1) vaporization and decomposition temperatures are close together; (2) a difference exists in the vaporization temperatures between the film material and the organic solvent; and (3) the vapor pressures are all very low. For example, for a liquid feed made by dissolving Ba(DPM).sub.2 or Sr(DPM).sub.2 in THF, the liquid phase of the solvent ranges in the region marked as "a" in FIG. 13, and the liquid or solid phase of the material ranges in "a+c". Therefore, when raising the temperature of the liquid through the region marked "c" to vaporize the liquid feed which is in "a", there is a danger that only the solvent is evaporated, causing the film material to precipitate out and resulting in plugging of a vapor passage or degradation in the feed quality due to compositional changes.
Therefore it is generally considered that, when vaporizing a liquid feed material, it is necessary to rapidly heat the liquid to bring it quickly to the high temperature region. A known type of vaporizer apparatus utilizes a technology of first preparing atomized mist with the use of an ejector nozzle or ultrasonic vibrator, and then heating the mist in a high temperature region to produce a vapor.
Also, depending on the type of film to be produced, it is sometimes necessary to supply the feed at infinitesimally slow rates to the film growing chamber. When using an ejector nozzle to atomize a liquid, a carrier gas is delivered at fairly high pressures and it is difficult to atomize a minute quantity of process liquid. When using an ultrasonic atomizer, it has been difficult to find an ultrasonic element which can operate at high temperatures required to vaporize the feed, so that it is difficult to carry out atomization under stable conditions. Also, both atomization and spraying processes require a large space, and there is a danger that some of the feed material can become stagnant in the space and become degraded or cannot be reliably delivered to the vaporizer apparatus.
Furthermore, it is desirable that the feed vaporization step be carried out just before the vapor enters into the processing chamber so as to make the vapor transport passage as short as possible, and it is desirable that the apparatus be efficient enough to vaporize a necessary amount of liquid feed in a small space. But the conventional apparatus based on a two-step process of atomizing followed by vaporization requires a large space and cannot easily be made into a compact unit.
Therefore, it is evident that there has been a need for improving the current system of treating difficult-to-vaporize materials in such a way that, even at infinitesimal rates of supply of liquid feed, the apparatus works efficiently and precisely so that the required vapor can be supplied to the vaporizer apparatus in a stable state.